1. Field of the Invention
The invention relates to a semiconductor device having a metal gate and manufacturing method thereof, and more particularly, to a semiconductor device having a metal gate and manufacturing method applied with a gate last process.
2. Description of the Prior Art
With a trend toward scaling down the size of the semiconductor device, work function metals are used to replace the conventional polysilicon gate to be the control electrode that competent to the high-K gate dielectric layer. The conventional dual metal gate methods are categorized into gate first process and gate last process. Among the two main processes, the gate last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-K gate dielectric layer and the metal gate, and thus gradually replaces the gate first process.
Please refer to FIG. 1, which is a schematic drawing illustrating a conventional semiconductor device having a metal gate fabricated by the gate last process. In the conventional gate last process, a dummy gate or a replacement gate is formed on a substrate 100 and followed by steps of forming a conventional metal-oxide semiconductor (MOS) transistor device 110 and forming an inter-layer dielectric (ILD) layer 120. Subsequently, the dummy/replacement gate is removed to form a gate trench. Then the gate trench is filled with work function metals required by different conductivity type. However, layer 130 such as barrier layer or strained stress layer is often formed in the gate trench before forming the metals. And each layer 130 reduces an opening width of the gate trench by forming overhangs as depicted by circle A in FIG. 1. The overhang problem makes it difficult to fill the gate trench with the work function metal layer 140. Serious overhang problem such as overhang merging or the metals layer 140 merging even results in a seam 160 in the gate trench and makes the filling metal layer 150 cannot be formed in the gate trench as desired. Eventually, the electrical performance of the transistor device 110 having the metal gate is deteriorated.